Zirconia toughening of glass-ceramic materials

ABSTRACT

A ceramic material suitable for packaging of large scale integrated circuits is produced by the process of forming a mixture of a powdered glass ceramic material which is a glassy precursor to cordierite ceramic material, formed by the steps which are as follows: 
     a. Mix tetragonal phase material selected from the group consisitng of zirconia or hafnia powder containing a stabilizing oxide compound selected from the group consisting of MgO, CaO and Y 2  O 3  and a glass frit powder or frit of a glassy precursor of cordierite glass ceramic to yield a suspension of solids. Preferably, a binder is included. 
     b. Disperse the suspended solids to yield a dispersion of the zirconia or hafnia with the stabilizing oxide compound and the glassy precursor. 
     c. Densify the dispersion of zirconia or hafnia with the stabilizing oxide compound and the glassy precursor by a sintering heat treatment at a temperature of about 840° C. to melt the glassy precursor into a viscous fluid at a temperature below the melting point of the zirconia or hafnia powder particles to yield a densified intermediate material with the zirconia or hafnia particles encapsulated in the molden glassy percursor. 
     d. Crystallize the densified intermediate material into a polycrystalline composite by heating at 900° C. to 950° C. 
     The process yields a ceramic material consisting of the tetragonal phase material encapsulated in crystalline cordierite glass ceramic material. 
     This invention is a continuation in part of application Ser. No. 07/146,455 filed on Jan. 21, 1988 now abandoned which was a continuation of Ser. No. 06/892,687 filed Aug. 1, 1986 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ceramic packages and ceramic packagingmaterials for large scale integrated electronic circuits, and moreparticularly to a composition of such ceramic packaging materialsadapted to increasing the toughness (resistance to fracture) of suchceramic materials.

2. Description of Related Art

Zirconia particles toughen a ceramic by either impeding the motion of apropagating crack, or absorbing or dissipating its energy. H. Ruf and A.G. Evans, "Toughening by Monoclinic Zirconia", J. Amer. Cer. Soc.,66(55) 328-332 (1983) and N. Claussen and M. Ruhle, "Design ofTransformation-Toughened Ceramics", Advances in Ceramics, Amer. Cer.Soc., (3) 137 (1981). The former includes crack bowing and deflectioneffects. The latter, termed transformation toughening, results from astress induced martensitic transformation of the zirconia from itstetragonal crystal structure to its monoclinic crystal structure. Thistransformation is accompanied by a 4% volume increase and a shear strainup to a maximum of 6%. The attainable toughening is dependent upon thevolume fraction of transformable tetragonal phase, size and distributionof the zirconia particles, the elastic constraining properties of thematrix and the size of the transformed zone around the crack. Claussenet al. supra; F. F. Lange, "Transformation Toughening", J. Mater. Sci.,(17), 235-239 (1982); and A. G. Evans and A. H. Heuer, "TransformationToughening in Ceramics: Martensitic Transformation in Crack Tip StressFields", J. Amer. Cer. Soc., 63 (5-6) 241-248 (1980), and R. McMeekingand A. G. Evans, "Mechanics of Transformation Toughening In BrittleMaterials", J. Amer. Cer. Soc., 65(5) 242-245 (1982).

Zirconia toughening has been demonstrated in a variety of crystallineceramic materials in the last decade. The earliest example, dating backover ten years, was in a two-phase zirconia ceramic consisting oftetragonal zirconia in a matrix of cubic zirconia. This material,described by R. Garvie, R. H. Hannink and R. T. Pascoe, "CeramicSteel?", Nature, 258(12) 703-704 (1975) was produced by a precipitationheat treatment. Since this original work, a number of workers havedemonstrated that the fracture resistance of many crystalline ceramicmaterials can be increased by incorporating in them particles oftetragonal zirconia, which transform to the monoclinic form when thematerial is fractured.

Claussen et al. showed that the incorporation of zirconia into analumina body increased its fracture toughness. Claussen et al. preparedtheir materials by the usual methods of ceramic forming in which powdersof alumina and zirconia were mixed together and then fired tosufficiently high temperatures that the material sintered (densified) toform a monolithic body.

U.S. Pat. No. 4,316,964 of Lange et al. for "Al₂ O₃ /ZrO₂ " describes azirconia toughened alumina ceramic prepared by using submicron powdersof Al₂ O₃ ZrO₂. "The composite powders were ball-milled with methanoland alumina balls in a plastic container and then dried. Densificationwas achieved by hot-pressing the powders for 2 hours at a temperatureselected to obtain small grain size and therefore favor the retention oftetragonal ZrO₂." The pressing temperatures in TABLE 1 were from 1400°to 1600° C. At Col. 5, line 18 it is stated, "The average grain size ofthe end member compositions which were hot-pressed at 1400° C. was about2 μm for the Al₂ O₃ and 0.5 μm for the ZrO₂." The ceramic is not a glassceramic and the pressing temperatures employed are excessive from thepoint ot view of the packaging of integrated circuits.

Stevens and Evans, "Transformation Toughening by DispersedPolycrystalline Zirconia", Br. Ceram. Trans. J. Vol. 83, 28-31 (1984)describes transformation toughening of alumina ceramics by volumeexpansion when tetragonal zirconia transforms to the monoclinic form. Itstates at page 28, "The phenomenon of transformation toughened ceramicsrelies on the volume expansion, 3-5% and shear strain ˜7% developed whentetragonal zirconia transforms to the monoclinic form. Toughening of aceramic host material is attained by retention of the tetragonalzirconia in a metastable state, the phase change to the monoclinic formbeing initiated by the tensile stress field of an advancing crack.Within a fixed distance of the crack tip, determined by the elasticstress field in its vicinity, any metastable tetragonal zironia willtransform and, as a result of the volume expansion and accommodatingshear stains, exerts a back stress on the crack. . . . "

U.S. Pat. No. 4,358,516 of Lange, for "Sodium Ion Conductor, SolidElectrolyte Strengthening with Zirconia", describes how theincorporation of transformable tetragonal zirconia could be used toincrease the resistance to fracture of a sodium ion conductor solidelectrolyte ceramic, β-alumina. For example, the addition of solidgrains of metastable tetragonal ZrO₂ with "a grain size less than about2 μm and has dissolved it in a rare earth oxide such as Y₂ O₃ . . . "(See abstract). The materials are added to the alumina to provideimproved fracture toughness.

As in U.S. Pat. No. 4,316,964 Lange, one can use additions of rare earthoxides, such as yttria, to control the formation of zirconia that isstable in the finished material in its tetragonal form.

For example, the addition of 15 vol. % zirconia to β"-Al₂ O₃ increasesthe fracture toughness, K_(c), from 3.0 to 3.8 MPam^(1/2) and thestrength from 147 to 414 MPa. See Stevens et al. supra. Alumina with 7.5vol. % zirconia shows an increase in K_(c) from 4.5 to 7 MPam^(1/2), andadding 17.5 vol. % zirconia to spinel increased the strength from 200 to500 MPa. (See Claussen et al. supra.)

Originally used to toughen zirconia ceramics and alumina, the use oftransformation toughening has rapidly been accepted as a way ofincreasing the fracture resistance (toughness) of all sorts of ceramicsincluding oxides, nitrides and carbides. In all these materials, markedincreases in toughening have been achieved when the processingconditions have enabled the tetragonal form of zirconia to be retainedin the microstructure. However, there is one class of materials inwhich, despite the incorporation of tetragonal zirconia, truetransformation toughening has not been reported, namely glasses andglass-ceramics.

Previous workers, who have attempted to toughen glasses andglass-ceramics by the use of zirconia, have done so by precipitating thezirconia from the glass phase. This is the manner in which zirconia, oneof the traditional nucleating aids in the manufacture of glass-ceramics,is known to be formed in its tetragonal form.

For instance, U.S. Pat. No. 4,396,682 of Mohri et al. for "GlazedCeramic Substrate" describes a ceramic print head with 50-60 wt. % ofSiO₂, 10-30 wt. % of CaO and MgO, and 2-6 wt. % of ZrO₂ plus optionalmaterials including the addition of one of BaO, ZnO, PbO, P₂ O₅, B₂ O₃,Na₂ O and K₂ O. The resulting material is a glass which has excellenthigh temperature stability for use in thermal print heads. The processused involves the heating in air to 1400° C., which is far above anacceptable level of heating for air process since it would be far abovethe melting point of the metallization. At Col. 3, lines 6-21 it isexplained that the ZrO₂ raises the transition point of the glasscomposition above 2 wt % of ZrO₂ in the material. Above 6 wt % the ZrO₂becomes an obstacle to the surface smoothness of the material. Nomention is made of the tetragonal phase of the zirconia. In view of thetemperature of 1400° C. to which the material is heated in the Example,the zirconia is in a solid solution in the glass. Moreover, no mentionis made therein of particles of zirconia in the glass.

See also U.S. Pat. No. 4,353,047 where zirconia is added as a nucleatingagent.

U.S. Pat. No. 4,234,367 of Herron, Master and Tummala for "Method ofMaking Multilayered Glass Ceramic Structures Having an InternalDistribution of Copper-based Conductors" describes use of cordieriteglass in conjunction with a thermoplastic binder in laminated greensheets. The glass comprises MgO, Al₂ O₃, SiO₂, B₂ O₃, SnO₂, Al₂ O₃, P₂O₅ and ZrO₂ glass particles in a glass ceramic. The laminate of greensheets is heated to a burnout temperature of 720° to 785° C. Then thelaminate is later heated to a crystallization temperature of about 920°to 970° C. Here the composition of the glass particles is quitedifferent from those employed here in view of the presence of P₂ O₅, B₂O₃ and in some cases SnO₂ and the absence of a stabilizing compound suchas yttria plus the absence of teachings of particle size limitationsclaimed herein.

U.S. Pat. No. 4,301,324 of Kumar, McMillan and Tummala for "GlassCeramic Structures of Gold, Silver or Copper" describes β-spodumeneglass ceramic and cordierite glass ceramic materials. In connection withthe cordierite glass. There is no CaO or Y₂ O₃ stabilizing material, butthere is CaO with respect to the β-spodumene at Col. 7, lines 61 and 62and in Table 1, Col. 4. The cordierite glass does contain from 0 to 2.5wt % of ZrO₂ in Table 1. Glass No. 11 in Table III includes B₂ O₃ plusZrO₂ but also includes P₂ O₅ and is sintered at 925° C. Glass No. 12 inTable III includes B₂ O₃ but also includes P₂ O₅ and is sintered at 950°C. There is no suggestion of using yttria as a stabilizing agent. Thereis also no mention of the tetragonal zirconia phase in connection withGlasses No. 11 and 12. The cordierite is of the μ form for glass No. 11and of the α form for glass No. 12. See Col. 9, lines 17 to 45. It isstated that the ceramic has greater strength than other ceramics. It isstated that it was thought that the enhanced strength was attributableto inclusion of ZrO₂, Col. 9, lines 25 to 28. There is no discussion ofthe particle sizes of the zirconia. A key distinction of the result ofthe process of the instant invention from the process of Kumar et al. isthat the zirconia is in a solid solution in the glass ceramic. We findthat there is no encapsulation of particles of zirconia in thetetragonal phase in the Kumar et al. product.

The objective of this invention is to provide a glass which istransformation toughened through the use of zirconia, which glass iscrystallized to a cordierite (2MgO.2Al₂ O₃.5SiO₂) glass-ceramic uponheat treatment. The method of manufacture, a "composite" route in whichparticles of the glass, having the desired glass-ceramic composition,and the zirconia are mixed together and fired as in a standard ceramicprocess. It avoids the conventional glass-ceramic manufacturing routewhich involves melting of the ingredients, and subsequentcrystallization to produce the glass-ceramic body. Furthermore, themethod enables the size of the tetragonal particles to be kept within adesirable range.

U.S. Pat. No. 4,421,861 of Claussen for "High-Strength andTemperature-Change Resistant Ceramic Formed Body, Especially of Mullite,its Production and Use" describes a zirconia toughened cordieriteceramic made by a reaction process, which involves deriving zirconiafrom a salt of zirconium. To obtain zirconia in that way the material issintered at unacceptably high temperatures of from 1300° C., to 1600°C., preferably. Such a high temperature is far higher than an acceptabletemperature for formation of VLSI packaging products since it is so highthat the copper metallization of the circuits on the package would bedestroyed by the heat. In short, the copper would turn to uselesspuddles on the package. Thus the Claussen et al. process is a verysignificantly different process from ours. It also produces a muchdifferent result.

Ruh et al. "Phase Relations in the System ZrO₂ -Y₂ O sub 3 Contents",Communications of American Ceramic Society, C-190 to C-192, (September1984), describes use of yttria with zirconia to lower themonoclinic-tetragonal transformation temperature of zirconia, but itdoes not suggest the use of such material with cordierite.

B. Schwartz "Making High Strength Ceramics", IBM Technical DisclosureBulletin, Vol. 11, No. 7,848 (December 1968), describes placing thesurfaces of a ceramic material in compression relative to the centralportions of the article by altering the composition of the outer layersof at least three layers of green ceramic material slightly prior tofiring, by adding chromium to alumina. The ceramic materials are used assubstrates for microelectronic devices. Obviously this disclosure doesnot contemplate use of zirconia or the equivalent as the material whichprovides hardening. In addition it does not suggest the temperaturerange that is taught here.

In D. J. Green and M. G. Metcalf, "Properties of Slip-CoatTransformation-Toughened β"-Al₂ O₃ / ZrO₂ Composites", Ceramic Bulletin,Vol. 63, No. 6 pp. 803-807, and 820 (1984), it is stated at page 805first full paragraph that "The majority particles are less than 1 μm forboth powders but there are some particles as large as 20 μm".

Porter, D. L., Evans, A. G., & Heuer, A. H. Acta MetalVol. 27, p. 1649(1979) describes toughening of β" Alumina and of Zirconia, respectively.None of the prior art suggests the range of sizes of particles ofzirconia or hafnia. The temperature range used in forming the hardenedceramic materials is suggested by none of the prior art for formingceramic, but merely to the formation of ceramics and the Herron et al.U.S. Pat. No. 4,234,367 does not relate to hardening of ceramics, perse.

A number of test methods have been used to measure the fracturetoughness of ceramics, and the effect of zirconia additions. One ofthese is the indentation test method, which is described in detail byAntis et al., "A Critical Evaluation of Indentation Techniques ForMeasuring Fracture Toughness: I. Direct Crack Measurements", Journal ofthe American Ceramic Society, 64(9) 533-538, (1981). In this method, adiamond pyramid is pressed, with a known load, into the surface of amaterial until cracks propagate from the corners of the indentationimpression. The length of the cracks so formed for a given load are ameasure of the resistance of the material to fracturing (its so-calledfracture toughness).

The transformation of zirconia from its tetragonal form to itsmonoclinic form as a result of the passage of a crack (the basis oftransformation toughening) has been shown to lead to an increase infracture toughness by Garvie et al., "Ceramic Steel?", Nature, (258),pp. 703-704 (1975) and by Clarke and Adar, "Measurement Of TheCrystallographically Transformed Zone Produced By Fracture In CeramicsContaining Tetragonal Zirconia", Journal Of The American Ceramic Society65(6) pp. 284-288 (1982). Garvie et al. measured their materials beforeand after fracturing by techniques of X-ray diffraction, to show thatsome of the tetragonal zirconia had transformed to monoclinic zirconia.Clarke and Adar used Raman spectroscopy to show that some of zirconiaparticles around cracks had transformed from the original tetragonalform to the monoclinic form.

The degree of transformation of zirconia from the tetragonal form to themonoclinic form dictates the attainable toughening. Thus, if only 25% ofthe tetragonal zirconia in a body is transformed to monoclinic zirconai,the fracture toughness will not exceed 25% of the theoreticallypossible. Likewise, a material made in such a way that part of thezirconia is already in its monoclinic form, there will be less zirconiaavailable in its tetragonal form for transformation to monoclinic.

SUMMARY OF THE INVENTION

In its broadest aspect this invention is a fracture toughened glassceramic, the method of fabrication thereof and a toughened glass ceramicsubstrate containing conducting patterns embedded therein.

There is a need to improve the strength of glass-ceramics. The materialneeds to be toughened in order to withstand handling in the electroniccircuit manufacturing process.

An object of this invention is to provide a process of manufacturingceramic materials suitable for packaging of electronic circuits at atemperature compatible with the metallization used to provide electricalcircuit conductors. Producing a ceramic laminate with conductor patternstherein is well known in the art. U.S. Pat. No. 4,234,367 to Herron etal. and U.S. Pat. No. 4,504,339 to Kamehara et al., the teachings ofwhich are incorporated herein by reference, are primarily directed toproducing such a laminate:

As taught in Herron et al.: In view of the high packaging densitiesattainable with multilevel ceramic circuit structure, they have achievedextensive acceptance in the electronics industry for packaging ofsemi-conductor integrated devices, and other elements, as for examplesee U.S. Pat. No. 3,379,943 granted Apr. 23, 1968 to J. G. Breedlove,U.S. Pat. No. 3,502,520, granted Mar. 24, 1970 to B. Schwartz and U.S.Pat. No. 4,080,414 granted Mar. 21, 1978 to L. C. Anderson et al. Ingeneral, such conventional ceramic structures are formed from ceramicgreen sheets which are prepared from ceramic "paints" by mixing aceramic particulate, a thermoplastic polymer (e.g. polyvinylbutyral) andsolvents. This "paint" is then cast or spread into ceramic sheets orslips from which the solvents are subsequently volatilized to provide acoherent and self-supporting flexible green sheet, which may be finallyfired to drive off the resin and sinter the ceramic particulatestogether into a densified ceramic substrate. In the fabrication ofmultilevel structures, an electrical conductor forming composition isdeposited (by spraying, dipping, screening, etc.) in patterns onrequired green sheets, which form component layers of the desiredmultilevel structure. The component sheets may have via or feedthroughholes punched in them, as required for level interconnection in theultimate structure. The required number of component green sheets arestacked or superimposed to register on each other in the required order.The stack of green sheets is then compressed or compacted at necessarytemperatures and pressures to effect a bond between adjacent layers noteseparated by the electrical conductor forming pattern. Thereafter, thegreen sheet laminate is fired to drive off the binders and to sinter theceramic and metal particulates together into a ceramic dielectricstructure having the desired pattern of electrical conductors extendinginternally therein.

We have found it is critically important that the process be performedat a temperature below the melting or sintering temperature of themetallization, such as copper, formed on or within the ceramic material.The melting point of copper is about 1083° C. If the metallization isheated excessively, it melts, disperses, or acts as a flux to the glassor melts to form a puddle on the ceramic material. If the metallizationis damaged, the electrical circuits in the integrated circuit structureare destroyed, thereby destroying the value of the package. Previouswork in ceramics did not involve such metallization and accordinglyprocess temperatures which were far too high have been employed. It is aprerequisite in the art of electronic circuit packaging that the ceramicmaterials be processed at lower temperatures compatible with preservingthe metallization structures on the packages. Metals for use in packagesinclude but is not limited to Ag, Au, Al and Cu.

Ceramic materials suitable for practicing the present invention includebut are not limited to cordierite, spodumene, eucryptite, borosilicateglass, lead glass, enstatite, celsian, wollastonite, willemite,anorthite, lithium disilicate, lithium metasilicate, mullite,combinations thereof and combinations thereof with alumina. This list isexemplary only and not limiting.

U.S. Pat. Nos. 4,301 and 4,413,061 both to Kumar et al., the teachingsof which is incorporated herein by reference, describe spodumene andcordierite compositions. The following is a list of the general formulafor the predominant components of the materials mentioned above:

celsian, BaO.Al₂ O₃.2SiO₂

anorthite, CaO.Al₂ O₃,2SiO₂

lithium disilcate, Li₂ O₆.2SiO₂

lithium metasilicate, Li₂ O.SiO₂

wollastinite, CaO.SiO₂

willemite, 2ZnO.S_(i) O₂

eucriptite, Li₂ O.Al₂ O₃.2SiO₂

mullite, 3Al₂ O₃.2SiO₂

enstatite, MgO.SiO₂

The term glass ceramic means an aggregate of randomly orientedcrystallites, for example, the material listed above, wherein theintersticies between crystallites may contain uncrystallized materialsuch as glass, for example the precursor of the above listed materials.

For convenience the invention will be described with reference to acordierite glass ceramic which is the most preferred glass ceramic.

In accordance with this invention, a ceramic material suitable forelectronic large scale integrated circuit packaging comprises acordierite or other glass ceramic material mixed with particlesconsisting essentially of a powdered, tetragonal phase of a materialselected from the group consisting of one or more zirconia or hafniapowder containing a stabilizing oxide compound. The cordierite or otherglass ceramic crystalline material encapsulates the particles and thestabilizing oxide compound. The stabilizing oxide compound is selectedfrom the group consisting of MgO, CaO, Y₂ O₃ and titania, and selectedrare earth oxides. The stabilizing oxide compound comprises from 0.1mole percent to 8 mole percent of said zirconia or hafnia. Thestabilizing oxide compound plus said zirconia of hafnia comprises atleast about 5 volume percent of the total volume percent of the ceramicmaterial. The glass ceramic material comprises at least about 75 volumepercent of the total volume of the ceramic material. The particles havea size within the range from about 0.5 to about 8.0 μm.

A ceramic material in accordance with this invention, which is suitablefor packaging, is produced by the process of forming a mixture of apowdered glass ceramic material which is a glassy precursor tocordierite or other ceramic materials, formed by the steps which are asfollows:

a. Mix tetragonal phase material selected from the group consisting ofzirconia or hafnia powder containing a stabilizing oxide compoundselected from the group consisting of MgO, CaO and Y₂ O₃ and a glassfrit powder or frit of a glassy precursor of cordierite or other glassceramics to yield a suspension of solids. Preferably, a binder isincluded.

b. Disperse the suspended solids to yield a dispersion of the zirconiaor hafnia containing the stabilizing oxide compound and the glassyprecursor.

c. Densify the dispersion of zirconia or hafnia containing thestabilizing oxide compound and the glassy precursor by a sintering heattreatment at a temperature above the glass transition temperature tomelt the glassy precursor into a viscous fluid at a temperature belowthe melting point of the zirconia or hafnia powder particles to yield adensified intermediate material with the zirconia of hafnia particlesencapsulated in the molten glassy precursor. The preferred temperatureis about 840° C.

d. Crystallize the densified intermediate material into a polycrystalinecomposite by heating up to about 1100° C., preferably from about 840° C.to about 950° C., most preferably, from 900° C. to 950° C. Thecrystallization time decreases with the temperature being for acordierite glass ceramic about 1 hour at 840° C. and about a minute at950° C.

The process yields a ceramic material consisting of the tetragonal phasematerial encapsulated in crystalline cordierite or other glass ceramicmaterials.

A ceramic material suitable for packaging is produced by the process offorming a mixture of a powdered glass ceramic material which is a glassyprecursor to the cordierite or other crystalline glass ceramicmaterials, formed by the steps which are as follows:

a. Mix zirconia or hafnia powder containing a stabilizing oxide compoundselected from the group consisting of MgO, CaO and Y₂ O₃ and a glassfrit powder or frit of the glassy precursor yielding a suspension ofsolids. Preferably, prior to mixing, perform the step of milling of thezirconia or hafnia powder in a fluid in a ball mill for one hour toproduce ball milled zirconia or hafnia powder. Preferably, the fluidused to facilitate mixing is methanol. Preferably, the step of ballmixing of the glassy precursor and the zirconia or hafnia mixture isperformed for a time duration of on the order of from about two minutesto about 13 hours, preferably 13 hours. The mixture is preferably mixedduring the dispersion step with an ultrasonic probe. Then it ispreferable that the product of mixing with an ultrasonic probe be driedwhile stirring magnetically or the equivalent.

b. Disperse the suspended solids to yield a dispersion of the zirconiaor hafnia with the and the glassy precursor.

c. Densify the dispersion of zirconia or hafnia and glassy precursor byheat treatment at a temperature of about 840° C. to yield a densifiedintermediate material.

d. Crystallize the densified intermediate material into apolycrystalline composite material by heating up to about 1100°preferably from about 840° C. to 950° C.

e. The process yields a ceramic material consisting of the tetragonalphase material encapsulated in crystalline cordierite or other glassceramic materials.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the X-ray diffraction pattern of 2.2 mol % Y₂ O₃ --ZrO₂ asreceived from supplier.

FIG. 2 shows the X-ray diffraction pattern of 10 vol % 2.5 mol % Y₂ O₃--ZrO₂ incorporated into the cordierite glass ceramic of TABLE I.

FIG. 3 shows the Fracture Toughness K_(c) vs Vol % ZrO₂ indicating thevariation of fracture toughness with the addition of pre-calcined 2.2mol % Y₂ O₃ --ZrO₂.

FIG. 4 is a graph of the fracture toughness of the glass-ceramic fordifferent values of yttria additions and for different volume additionsof zirconia.

FIG. 5 is a sketch based upon a photograph of a Vickers indentation froma 88.236N load in a Table I composition of cordierite glass-ceramiccontaining 10 vol % 2.5 mol % Y₂ O₃ --ZrO₂.

In FIG. 6, are presented Raman Spectra recorded using an optical probeof a material shown in TABLE III.

The foregoing and other objects, features and advantages of theinvention will be apparent from the more particular description of thepreferred embodiments of the invention which follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT Disclosure of the Invention,Best Mode and Other Modes of Carrying Out the Invention

Transformation toughening requires a well dispersed phase of metastabletetragonal zirconia, that upon fracturing will transform to themonoclinic form. Zirconia exists as three phases, monoclinic, tetragonaland cubic. Monoclinic is the stable form below 950° C. The monoclinic totetragonal transformation occurs between 950° C. and 1200° C. The cubicphase is formed at 2370° C. as explained by R. Stevens, Introduction toZirconia, Magnesium Elecktron Inc., Flemington, N.J. (1983). Tetragonalzirconia is retained at room temperature through the use of stabilizingoxide compounds such as MgO, CaO and Y₂ O₃, control of the particle sizeand the elastic properties of the host matrix. (See Lange, supra andStevens et al., supra). Adding the stabilizing oxide compounds magnesia(MgO), calcia (CaO) and yttria (Y₂ O₃), reduces the tetragonal tomonoclinic transformation temperature.

Zirconia or hafnia is added to the frit of a glass precursor of acordierite glass-ceramic to toughen the fabricated glass ceramic.Zirconia or hafnia is added in the tetragonal phase. This phase isunstable at temperatures below about 950° C. The tetragonal phasetransforms to the monoclinic form at temperatures above about 950° C. Attemperatures below 950° C. crack formation in the ceramic initiates thetransformation from tetragonal to monoclinic crystal structure. Thetransformation involves a volume expansion which causes the toughening.Toughening increases as the percentage of the tetragonal phaseincreases. When the ceramic material is used as a microelectronicpackaging substrate, the use temperature is generally less than about200° C. If the transformation temperature is closer to the usetemperature, a larger percentage of the zirconia or hafnia will be inthe tetragonal phase since it is more stable with a lower transformationtemperature. The transformation temperature is lowered by adding to thezirconia or hafnia a stabilizing oxide material. A molecule of thestabilizing oxide takes the place of a zirconia or hafnia molecule at alattice site of the tetragonal crystal structure. This substitutionresults in lowering the transformation temperature. To achieve effectivestabilization, a fraction of the total lattice sites must be replaced bystabilizing oxide molecules. As used herein, stabilizing refers tolowering the tetragonal to monoclinic zirconia transformationtemperature. This is achieved by adding stabilizing oxides to thetetragonal zirconia.

We have found that the yttria stabilized zirconia, employed inaccordance with this invention, forms the transformable tetragonal phasein the compositional range of 0-5 mol %. Decreasing the particle sizealso increases the stability of the tetragonal phase. The maximumparticle size at which pure zirconia will remain tetragonal is generallyless than 1 μm, whereas for a 2 mol. % Y₂ O₃ stabilized zirconia it islarger. The volume constraining effects of the matrix also increases thestability of the constraining effects of the matrix also increases thestability of the tetragonal form, increasing the critical diameter. (SeeLange, supra). Pure zirconia particles less than 0.5 μm in diameterremain tetragonal while constrained in an alumina matrix. The criticaldiameter in a matrix increases with the use of the partially stabilizedzirconias of MgO, CaO and Y₂ O₃.

Dispersion of zirconia powder in the host matrix is an importantprocessing step of this invention. Agglomerates of zirconia particlescontribute flaws to a fired microstructure. Techniques for dispersionincluded mechanically mixing the powders, attrition of zirconia grindingmedia, and sol-gel techniques. (Claussen et al., supra). Aside frommechanical mixing, the other processing techniques are expensive anddifficult to control. For these reasons, in one aspect of thisinvention, the mechanical mixing approach is employed in accordance withthis invention.

Examples of compositions of glass ceramic materials toughened accordingto the present invention, suitable for electronic large scale integratedcircuit packaging, are shown in TABLE I.

                  TABLE I                                                         ______________________________________                                        WEIGHT PERCENT                                                                CORDIERITE                                                                    +3%           BETA-        WILLE-   ANOR-                                     ENSTATITE     SPODUMENE    MITE     THITE                                     ______________________________________                                        Al.sub.2 O.sub.3                                                                    21.23        14.42       14.0    14.42                                  MgO   20.00       --           --     --                                      SiO.sub.2                                                                           55.00       71.5         31.0   55.0                                    P.sub.2 O.sub.5                                                                      2.77        2.08        --      2.08                                   B.sub.2 O.sub.3                                                                      1.00       --           10.0   --                                      LiO.sub.2                                                                           --          10.0         --     --                                      K.sub.2 O                                                                           --           2.0         45.0   --                                      ZnO   --          --           --     --                                      CaO   --          --           --     23.0                                    ZrO.sub.2                                                                           --          --           --      2.5                                    ______________________________________                                    

The most preferred glass ceramic contains a cordierite glass ceramic, ofthe formula (2MgO--2Al₂ O₃ --5SiO₂) encapsulating particles of zirconia,hafnia or some combination of the two such as an alloy thereof. Theseparticles increase in size when they transform from their initialcrystal structure to a structure which requires more volume. Thisincrease in volume produces forces which increase the fracture toughnessof the material.

We have discovered that it is essential that the zirconia or hafniaparticles have a size within the range from about 0.5 μm to about 8.0 μmand preferably about 3.0 μm. It is critically important to theinvention, that the particles are so small. When materials are made withsignificantly larger sizes than the range from about 0.5 μm to about 8.0μm, no transformation toughening has been obtained during ourexperiments with the process of this invention.

In addition, we have found it is critically important that the processbe performed at a temperature below the melting or sintering temperatureof the metallization such as copper formed in, i.e. on or within, theceramic material. If the copper is heated excessively, it melts,disperses, or acts as a flux to the glass or melts to form a puddle onthe ceramic material. If the copper is damaged, the electrical circuitsin the integrated circuit structure are destroyed, thereby destroyingthe value of the package.

In this invention the known additive of a stabilizing oxide compoundmaterial, such as yttria, magnesia, calcia, titanium dioxide and oxidesof rare earths, is employed to reduce the tetragonal to monoclinictransformation temperature so that the tetragonal state will bemaintained at room temperature. Yttria is sometimes included within theclass of rare earth oxides. However, yttria has atomic number 39,whereas the lathamide rare earth elements have atomic number 58 to 71and the actinide rare earth elements have atomic number 90 to 101.

EXAMPLES

Five zirconia powders have been employed. One was a pure unstabilizedzirconia with a mean particle diameter of 0.03 μm and a partiallystabilized 2.5 mol. % yttria-zirconia with a mean size of 0.02 μm. Thethird, a 2.2 mol. % yttria-zirconia with a mean size of 25 μm wasemployed.

Two 3 mol. % MgO-zirconia powders were investigated, one of which wasspray dried. These were prepared from the citrates.

X-ray diffraction studies revealed that the tetragonal content of thepartially stabilized powders could be increased by calcining (heating agranular or particulate solid at a temperature sufficient to remove mostof its chemically combined volatile matter) to 1180° C. Samples wereprepared using both the original powders as received from themanufacturers and powders calcined, as above.

Ball-Milling

Ball-milling was used to disperse the zirconia powders with the glasspowder of the glass ceramic material shown in TABLE I, above.

For compatibility with current techniques for substrate manufacture,methanol, used in the green sheet binder system for MLC structures, isemployed as the milling suspension. Methanol produced a superiordispersion of zirconia powders when compared with methyl iso-butylketone.

The zirconia powders were milled for one hour prior to adding the powderof the composition shown in TABLE I. Then they were milled an additional13 hours.

After milling, the powders were further dispersed with a 300 wattultrasonic probe operated at maximum power for two minutes.

Evaporation of Solvent

After the ultrasonic mixing step, the methanol solvent was evaporatedfrom the samples while mixing with a magnetic stirrer to preventpreferential particle settling of the denser zirconia particles.

Pressing

To reduce porosity to a minimum for accurate fracture toughnessmeasurements, one gram green pellets were uniaxially pressed at 88 MPa(Megapascals, where 1 Megapascal=145 psi), without binder, and thenisostatically pressed at 69 MPa.

Air Firing

All samples were subsequently air-fired at 960° C. for two hours.

The fracture toughness of the samples was measured using the indentationtechnique after the sample surfaces were ground and polished. See G. R.Anstis, P. Chantikul, B. R. Lawn and D. B. Marshall, "A CriticalEvaluation of Indentation Techniques for Measuring Fracture Toughness:I, Direct Crack Measurements", J. Amer. Cer. Soc. 64(9) 533-538 (1981);B. H. Mussler and M. W. Shafer, "Preparation and Properties ofMullite-Cordierite Composites", Bull. Amer. Ceram. Soc., 63 (5) 705-710(1984).

X-ray diffraction revealed that the polished and as fired surfaces wereequivalent in tetragonal phase content, assuring test validity. At leastthree indentations per sample were made with a Zwick (Zwick Company OldSaybrook, Conn.) hardness tester, configured with a Vickers diamondpyramidal indenter. Indentations were done at loads of 29.412 and88.236N.

The sonic resonance technique (Schreiber, Anderson and Soga, ElasticConstants and Their Measurements, McGraw-Hill, New York, p. 91 (1974))was used to measure Young's modulus (required for fracture toughnesscalculations) for a bar of 10 vol % 2.5 mol % yttria-zirconia and thematerial of TABLE I, above, air fired to 960° C. This bar was made bythe lamination at 31 MPa, of 15 layers of 0.29 mm cast green sheets. Thegreen sheet slurry was prepared by using 130% of the binder systememployed for casting the powder of TABLE I. Due to the high surface areaof the 0.02 μm zirconia particles, more binder liquid was needed tolower the slurry viscosity in order to pour it from the ball mill.Dispersion was done in the binder system, first by milling the zirconiapowder for one hour and then adding the powder of TABLE I and ballmilling for an additional two hours. It should be noted that this methodis not effective for complete dispersion and was used only for a Young'smodulus specimen.

FIG. 1 shows the X-ray diffraction pattern of 2.2 mol % Y₂ O₃ --ZrO₂ asreceived from the supplier.

FIG. 2 shows the X-ray diffraction pattern of 10 vol % 2.5 mol % Y₂ O₃--ZrO₂ incorporated into the cordierite glass ceramic of TABLE I.

X-ray analysis of the powders showed that calcining the commerciallymanufactured, partially stabilized zirconia to 1180° C. couldsignificantly increase the tetragonal phase content. The results, shownin TABLE II below indicate an increase from 50% to 95% tetragonal phasefor the 2.5 mol % yttria-zirconia of sample 1 in TABLE II and from 73%to 85% for the 2.2 mol % yttria-zirconia of sample 9 in TABLE II below.The pure zirconia remained monoclinic.

                  TABLE II                                                        ______________________________________                                        X-RAY DIFFRACTION DATA                                                        VOL % TETRAGONAL                                                              SAMPLE          POWDERS    FIRED PELLETS                                      ______________________________________                                        1)  MgO--ZrO.sub.2 (Sp. dried)                                                                     74%        10%                                           2)  Calc. 1180° C.                                                                          0         .sup. NM*                                      3)  MgO--ZrO.sub.2 (Sp. dried)                                                                     42%        0                                             4)  Calc. 1180° C.                                                                          0         .sup. NM*                                      5)  Pure ZrO.sub.2   0          0                                             6)  Calc. 1180° C.                                                                          0         .sup. NM*                                      7)  2.5 mol % Y.sub.2 O.sub.3 --ZrO.sub.2                                                         50         58                                             8)  Calc. 1180° C.                                                                         95         66                                             9)  2.2 mol % Y.sub.2 O.sub.3 --ZrO.sub.2                                                         73         58                                             10) Calc. 1180° C.                                                                         85         .sup. NM*                                      ______________________________________                                         *NM = Not Measured                                                       

Chemically prepared 3 mol % magnesia-zirconia showed a reversal intetragonal content, becoming completely monoclinic upon calcination. Thespray dried powder did show the highest tetragonal content, 74%, of theas-received powders. The magnesia-zirconia system does decompose at1200° C. An analysis of a sample of this zirconia dispersed in theglass-ceramic and fired to only 960° C. shows that the tetragonalcontent is very low compared to other specimens containing partiallystabilized zirconia (TABLE II). It is possible that various cationicimpurities are present in these citrated derived powders that lendstability to the tetragonal phase. Upon firing, these impurities areevolved, decreasing the stability of the tetragonal phase. If thestabilizing oxide compounds along with the impurities could becontrolled such that the tetragonal phase begins to lose stability at960° C., one could incorporate a highly metastable tetragonal zirconiain the glass-ceramic matrix. One would have a zirconia powder thatremains highly tetragonal during processing at room temperature, butbecomes highly metastable upon heat treatment to 960° C., maximizingtransformation toughening.

The zirconia powders partially transform upon processing. A precalcinedpowder that was 95% tetragonal changed to 74% tetragonal after attritormilling for 30 minutes. The formation of the green pellets by pressing,did not lead to transformation of the zirconia particles.

The fired samples of the commercially manufactured zirconiasincorporated in the glass-ceramic do show considerable tetragonalcontent in an X-ray diffraction pattern, FIG. 2. The tetragonal contentcalculated for these samples are rough estimates and should not bedirectly compared with data obtained for zirconia powders.

Additional test firing the zirconia mixed with the glass of TABLE Irevealed that no zircon (ZrSiO₄) was formed upon firing the zirconiamixed with the glass of TABLE I even up to temperatures of 1180° C.,holding the temperature there for one hour.

Young's modulus for a composite of 10 vol % 2.5 mol % yttria-zirconia inthe material of TABLE I, air fired to 960° C., as determined by sonicresonance was 137 GPa. This compares with a Young's modulus of 130 GPafor the material of TABLE I alone. The 137 GPa value coincides with thecalculation of Young's modulus from equations for two phase systems.Young's modulus for 15% added zirconia was not measured, but calculatedas 141 GPa.

The results of fracture toughness measurements are given in both TABLEIII and in the graph of FIG. 3. FIG. 3 shows the Fracture ToughnessK_(c) vs Vol % ZrO₂ indicating the variation of fracture toughness withthe addition of precalcined 2.2 mol % Y₂ O₃ --ZrO₂. The maximum error inthese measurements is 0,1 MPam^(1/2). The values for fracture toughness,K_(c), were all measured at the same load, of 88.236N (9 Kg). The graphof FIG. 4 plots the fracture toughness of the glass-ceramic fordifferent values of yttria additions and for different volume additionsof zirconia. It plots K_(c) in MPam^(1/2) vs the Zirconia type used for15 and 20 percent zirconia with 3, 4 and 5 percent yttria. The variationin measured values of fracture toughness exhibited by the data of FIG. 4was attributed to variations in dispersion of the zirconia particles andlocal densification resulting from the presence of agglomerates. Thevariation thus emphasizes the necessity of good dispersion of thezirconia particles during the material preparation. The increase infracture toughness of the materials containing zirconia, could be shownto be due to transformation toughening by examining the materials afterfracturing.

In FIG. 6, are presented Raman Spectra recorded using an optical probeof a material shown in TABLE III. The Raman Spectra were recorded from aregion remote an indentation crack (top) and from an indentation crack(bottom). The lower ration of the tetragonal to monoclinic peaks in thebottom spectrum indicates that a fraction of the the tetragonal zirconiagrains have been transformed by the fracturing process.

                  TABLE III                                                       ______________________________________                                        FRACTURE TOUGHNESS DATA                                                                           K.sub.C                                                                       (MPam.sup.1/2) ±                                                                      %                                              SAMPLE              0.1 Max    CHANGE                                         ______________________________________                                        1)  TABLE 1 COMPOSITION 1.75       --                                         2)  10 vol. % Pure ZrO.sub.2                                                                          2.00       +16                                        3)  10 vol. % 2.5 mol % 2.15       +23                                            Y.sub.2 O.sub.3 --ZrO.sub.2                                               4)  same-precalcined ZrO.sub.2                                                                        2.15       +23                                        5)  10 vol. % 2.5 mol % 2.43       +39                                            Y.sub.2 O.sub.3 --ZrO.sub.2                                               6)  same-precalcined ZrO.sub.2                                                                        2.30       +30                                        7)  15 vol. % 2.2 mol % --         --                                             Y.sub.2 O.sub.3 --ZrO.sub.2                                               8)  same-precalcined ZrO.sub.2                                                                        2.68       +53                                        9)  Beta-spodumene composition of                                                                     1.5        +20                                            TABLE I 15 Vol % ZrO.sub.2                                                    stabilized with Y.sub.2 O.sub.3                                           ______________________________________                                    

FIG. 5 is a sketch made from a photograph of a Vickers indentation froma 88.236N load in a TABLE I composition of cordierite glass-ceramiccontaining 10 vol % 2.5 mol % Y₂ O₃ -ZrO₂.

It is important that the zirconia particles be uniformly distributedthroughout the matrix. This powder did result in the highest value forfracture toughness, and should contribute the fewest flaws to the firedmicrostructure.

Adding 10 vol % of the 2.5 mol % yttria-zirconia to the glass-ceramicincreased its dielectric constant from 5.0 to 5.8. The value of 5.8compares with a calculated value of 5.5. The coefficient of thermalexpansion increased from 1.8×10⁻⁶ /°C. for the glass-ceramic alone to2.5×10⁻⁶ /°C. at 25° C. to 300° C.

Conclusions

1. Adding a second phase of zirconia particles to the glass-ceramic ofTABLE I reduces its brittleness and increases its fracture toughness.

2. Dispersion is very important to the densification and thus thestrength of this two phase system. The larger particles size, 25 μmpowder disperses very well during comminution and mixing in the ballmill and showed the highest fired density of the zirconia plusglass-ceramic mixtures. This powder also resulted in the highest valuesfor fracture toughness: adding 10 vol % increased K_(c) by 30% to 40%,and adding 15 vol % increased K_(c) by more than 50%.

3. The 0.02 μm and 0.03 μm powders were too fine to obtain a gooddispersion using this process.

ALTERNATIVE DESIGNS

In one alternative design, the zirconia or hafina particles are foundlocalized in a specific region, for example, only in the outer layer ofthe material to provide compressive forces there to form an outercompressive layer to resist crack propagation due to the transformationof the zirconia and/or hafina material to the monoclinic phase from thetetragonal phase, if possible. The incorporation of the zirconia only alocalized region such as in the outer layer of the ceramic has theadvantage of maintaining a low overall dielectric constant for theceramic material. That is in spite of the fact that the dielectricconstant of the zirconia and/or hafnia.

The ceramic material is produced by the process of forming a mixture ofa powdered glass ceramic material (which is a glassy precursor ofcordierite crystalline ceramic material), formed by the steps which areas follows:

1. Mix a powdered, tetragonal phase of a material selected from thegroup consisting of one or more of zirconia or hafnia powder containinga stabilizing oxide compound selected from the group consisting of MgO,CaO, Y₂ O₃, titania, and selected rare earth oxides, and a glass fritpowder or frit of a glassy precursor of a crystalline ceramic material,for example, cordierite, spodumene, eucriptite, borosilicate glass, leadglass, enstatite, celsian, wollastinite, willemite, anorthite, lithiumdisilicate, lithium metasilicate, mullite, combinations thereof andcombinations thereof with alumina, most preferably of a cordieritecontaining composition yielding a suspension of solids.

2. Disperse the suspended solids to yield a dispersion of the zirconiaor hafnia and the glassy precursor.

3. Densify the dispersion of zirconia or hafnia and glassy precursor bya sintering heat treatment at a temperature of about 840° C. to melt theglassy precursor composition into a viscous material at a temperaturebelow the melting point of the zirconia or hafnia powder particles toyield a densified intermidediate material with the zirconia or hafniaparticles encapsulated in the molten glassy precursor.

4. Crystallize the densified intermediate material by heating at 840° C.to 950° C.

A process is provided for making the new ceramic material suitable forpackaging. It is produced by the process of forming a mixture of apowdered glass ceramic material which is a glassy precursor to thematrix material of the ceramic matrix, formed by the steps which are asfollows:

1. Mix a powdered, tetragonal phase of a material selected from thegroup consisting of one or more of zirconia or hafnia powder, astabilizing oxide compound selected from the group consisting of MgO,CaO, Y₂ O₃, titania and selected rare earth oxides, and a glass fritpowder or frit of a glassy precursor of a crystalline ceramic materialmost preferably of cordierite composition yielding a suspension ofsolids. The stabilizing oxide compound comprises from 0.1 mole percentto 8 mole percent of the zirconia or hafnia.

2. Disperse the suspended solids to yield a dispersion of the zirconiaor hafnia containing a stabilizing oxide compound, and the cordierite.

3. Densify the dispersion of zirconia or hafnia and cordierite or otherglass ceramics by heat treatment at a temperature of about 840° C. toyield a densified intermediate material.

4. Crystallize the densified intermediate material into apolycrystalline composite by heating at 900° C. to 950° C.

In one version of this invention, prior to the step of mixing, one millsthe zirconia or hafnia powder in a fluid in a ball mill for one hour toproduce ball milled zirconia or hafnia powder. The fluid used tofacilitate mixing is methanol. Preferably, the step of ball mixing ofthe glassy precursor and zirconia or hafnia mixture lasts on the orderof 13 hours, and the mixture is mixed during the dispersion step with anultrasonic probe. Then one dries the product while stirring magneticallyor the equivalent.

INDUSTRIAL APPLICABILITY

This invention is applicable in data processing such as personalcomputers, minicomputers, large scale computers and other dataprocessing equipment. In addition, this system and process will beapplicable to industrial and consumer electronic devices employing LSIchips. Electronic products such as transportation and control systemsincorporating data processing systems for continuous monitoring and likefunctions can use the packaging methods and systems of this invention.

While the invention has been illustrated and described with respect topreferred embodiments, it is to be understood that the invention is notlimited to the precise constructions herein disclosed, and the right isreserved to all changes and modifications coming within the scope of theinvention as defined in the appended claims.

Having thus described our invention, what we claim as new, and desire tosecure by Letter Patent is:
 1. An article at least a part of whichcomprises:a ceramic body consisting essentially of at least about 75volume percent of a dielectric glass ceramic and the remainder being afinite amount of a material selected from the group consisting of ZrO₂,HfO₂ and a mixture thereof in the form of particles having a sizebetween about 0.5 and about 8.0 microns, said particles having atetragonal crystalline structure and being sufficiently dispersed withinsaid ceramic body to increase the fracture toughness of said ceramicbody; said material further containing an oxide compound selected fromthe group consisting of MgO, CaO, Y₂ O₃, TiO₂ and rare earth oxides,there being at least enough of said oxide to stabilize said tetragonalstructure of said material, and an electrical conductor pattern withinsaid ceramic body, said conductor pattern having a melting temperaturegreater than the crystallization temperature of said glass ceramic. 2.The article of claim 1, wherein said conductor pattern contains copper.3. The article of claim 1, wherein said ceramic body is at least a partof a surface of said article.
 4. The articles of claim 1, wherein saidglass ceramic is a cordierite glass ceramic.
 5. The article of claim 1,wherein said oxide compound is from about 0.1 to 8.0 mole percent ofsaid material.
 6. The article of claim 1, wherein said conductor patternhas a melting temperature less than about 1100° C.
 7. The article ofclaim 1, wherein said glass ceramic contains crystal phases selectedfrom the group consisting of cordierite, spodumene, eucryptite,enstatite, celsian, wollastonite, willemite, anorthite, lithiumdisilicate, lithium metasilicate, mullite, combinations thereof, andcombinations thereof with alumina.
 8. The article of claim 1, whereinsaid crystallization temperature is a specific temperature less thanabout 1100° C. and wherein said melting temperature of said conductorpattern is greater then said specific temperature.
 9. The article ofclaim 2, wherein said crystallization temperature is less than about1100° C.
 10. The article of claim 7, wherein said conducter patterncontains copper.
 11. The article of claim 10, wherein saidcrystallization temperature is less then about 1100° C.
 12. An articleat least a part of which comprises:a ceramic body consisting essentiallyof at least about 75 volume percent of a cordierite glass ceramic andthe remainder being a finite amount of a material selected from thegroup consisting of ZrO₂ HfO₂ and a mixture thereof in the form ofparticles having a size between about 0.5 to 8.0 microns, said particleshaving a tetragonal crystalline structure and being sufficientlydispersed within said ceramic body to increase the fracture toughness ofsaid ceramic body; said material further containing an oxide compoundselected from the group consisting of MgO, CaO, Y₂ O₃, TiO₂ and rareearth oxides, there being at least enough of said oxide to stabilizesaid tetragonal structure of said material; and an electrical conductorpattern, within said ceramic body, having a melting temperature greaterthan the crystallization temperature of said glass ceramic.
 13. Thearticle of claim 9, wherein said glass ceramic contains crystal phasesselected from the group consisting of cordierite, spodumene, eucryptite,enstatite, celsian, wollastonite, willemite, anorthite, lithiumdisilicate, lithium metasilicate, mullite, combinations thereof, andcombinations thereof with alumina.